For utilizing a hollow spherical particulate material, such as a calcined natural glass sand (Shirasu-balloons), for producing boards and blocks, a process is necessary for shaping (molding) it, while bonding the particles using a binding material. As the binding material, for example, resins, cements and sodium silicates have found their application therefor. Use of resins will result in a reduction of non-inflammability of the product, while use of cements may impair the contemplated light-weighing property and processibility of the product and use of sodium silicates may bring about inferior strength and lower processibility of the product.
A technique of producing composite boards has been known (Japanese Patent Application No.320967/1994), which comprises charging a mold cavity in a form of a plate with a hollow spherical particulate material and filling up the interstices of the charged particles with a molten metal, such as aluminum, to be used as the binding material, by impressing the molten metal into the mold cavity under a pneumatic pressure to soak the charged hollow particulate material. This technique has, however, defects that the dimensions of the composite board to be produced are limited to the range in which the flowability of the molten metal for soaking the hollow particles in the mold cavity can be maintained and that there is a large irregularity in the degree of filling up of the particle interstices with the metal over the product board, so that, even approximately, a homogeneous filling up and, therefore, uniformity of the material properties of the product board, cannot be achieved. These defects become increased when the board has greater extension or is thinner. For these reasons, this technique has not been in practice for the production of boards.
Processes of prior art for continuous production of uniform composites of inorganic material and metal are reviewed in our prior patent application (Japanese Patent Application No. 320967/1994, see above). A continuous production of boards may permit to expect a reduction in the production cost due to the high productivity, nevertheless there is a limitation in the design of the ornamental surface profile pattern on the board required for commercializing such boards. Thus, it is not able to produce a composite board having a surface profile of, for example, an undercut with a projection protruding rectangularly to the direction of sliding of the composite board along the inside face of the metal mold upon the production thereof. While it may be considered that a surface profile can be brought about by a roller embossing or a plate embossing after the composite board has been molded, such a measure may deteriorate the ornamental effect of the composite board due to the possible break down of the hollow particles of the inorganic material upon the embossing, since these hollow particles appear even within a quite shallow depth in the skin layer of the composite. For these reasons, the surface profile patterns achieved by the conventional continuous production techniques are limited to those in which the embossing patterns are quite shallow, e.g., hair-thick lines and the like, and run parallel with a constant width only in the direction of sliding of the composite board performed in the production thereof.
For producing a board or a block from hollow particulate material, the particles must be bonded rigidly together by, for example, using a binder. For such a binder, resins, cements, sodium silicates and others are used. Use of a synthetic resin for such a binder will result in a poor non-inflammability of the product. Use of cement will lead to a heavy-weighing product with poor processibility and use of sodium silicates brings about a product with lower strength and inferior processibility.
A technique has been known for producing a composite product from a hollow particulate material, which comprises filling up a plate-shaped mold cavity of a metal mold with the hollow particulate material and injecting therein a molten metal, such as aluminum, by impressing it by a pneumatic or a mechanical means to penetrate through the interstices between the particles, in order to bind the particles rigidly with each other by the metal as a medium for binding the particles.
This technique can only be applied to the case where the thickness of the product board allows a free flow of the molten metal to penetrate into the interstices between the particles. Thus, an allowable charging ratio of the molten metal to the particulate material, namely, a permissible uniformity in the material properties of the product composite can only difficulty be achieved.
The difficulties of the prior techniques as given above become more and more larger for products of greater size and lower thickness. For this reason, these techniques are still not in practical use for the production of composite board.
A technique has also been contrived, in which a molten metal is injected into a horizontally laying plate-shaped aggregation of a hollow particulate material in a horizontal mold from above to produce a composite impregnated with the metal. Here, the mold is provided at a portion thereof with a preheater to preheat the plate-shaped aggregation. However, due to the relationship between the geometry of the mold and the possible sliding velocity of the plate-shaped aggregation, it is difficult to choose a high preheating temperature of the aggregation of the particulate material. If the preheating temperature too low, it is necessary to increase the pressure for injecting the molten metal into the aggregation in order to attain penetration into the interstices between the particles. This pressure calculates to be doubled as compared with the case of bilateral injection of the molten metal, since the penetration depth is doubled. If the injecting pressure is increased, the proportion of collapse of the hollow particles due to compression increases, resulting in an increase in the apparent density of the composite board obtained. A higher injecting pressure brings about a further defective effect of necessitating an increase in the strength of the molten metal storage vessel. A disadvantageous measure may be necessary for evading these defective effects, for example, by incorporating a greater heat source, in order to attain a prompt preheating of the hollow particulate inorganic material.
Various processes have been contrived for producing a composite of metal and hollow particles of inorganic material and some have found their practical applications. All of these prior processes are fundamentally based on the principle of compressingly impregnating the particles with a molten metal in a static condition. The compressive impregnation becomes more difficult, as the thickness of the particle layer increases and the path for the penetration of the molten metal becomes longer. A principal cause of such increase in the difficulty of impregnation may be the circumstances that the stream of the molten metal upon penetration into the aggregation of the particles on passing through the quite narrow interstices between the particles will be branched into thin streamlets of very small diameters which may be liable to a rapid quench and solidification, losing thus rapidly their fluidity and blocking up the flow. If this difficulty is overcome by forcing the molten metal to penetrate the aggregation of the particles by impressing it into the interstices between the particles, some bulky streams of the molten metal will appear up to some depth by wedging the hollow particles away, before they branch into dendritic very thin streamlets. Within the bulky stream, the hollow particles may be collapsed and the molten metal fills up the particle inside to deteriorate the characteristic feature of light-weighing of the composite and there is a large local inequality in the material properties between the portion of the bulky stream and the portion of the dendritic streamlets. In order to obviate these disadvantages, a new technique was proposed and a corresponding patent application was made (Japanese Patent Application Ser. No. 320967/1994), in which a molten metal is injected into a board-like aggregation of the hollow particulate inorganic material held in the mold cavity, through perforations of a perforated plate which covers only a restricted area of one side face of the mold cavity, by pneumatically impressing the molten metal with a relatively low compressive force (1.5-2.5 kgf/cm.sup.2) so as to attain the penetration of the molten metal in the direction of thickness of the board in which the path of penetration is at the smallest. This technique has been proven experimentally to be a process for producing a composite board of a metal and a hollow particulate inorganic material exhibiting the lowermost local inequality in the material properties. This material property inequality which is the principal disadvantage of this technique increases, however, in proportion to the thickness of the bulk of aggregation for a composite product of a thick aggregation, such as a block, cylinder of larger diameter or the like, since the molten metal permeates through the interstices between the particles in the aggregation. It has therefore been expected to provide a method for producing composite products of a metal and a hollow particulate inorganic material with greater bulk thickness, by which such a local inequality in the material properties of the resulting composite product is minimized.